1. Field of the Invention
The present invention relates generally to an exciting coil drive circuit of a magnetic sensor, and more particularly, to technology for providing a highly-accurate and stable exciting coil drive circuit of a magnetic sensor.
2. Description of the Related Art
A so-called flux gate magnetometer is known which saturates a magnetic flux by inputting periodical drive signals to an exciting coil wound on a soft magnetic core to measure intensity of an external magnetic field from saturation time intervals changed depending on the size of the external magnetic field that is measured. The flux gate magnetometer has various excellent features for a magnetometer, such as (1) high sensitivity and magnetic field resolution, (2) capability of measuring a weak magnetic field, (3) a wide measurement range, (4) temperature stability better than magnetometers of other modes, and (5) high linearity to an input magnetic field.
Japanese Patent Application Laid-Open Publication No. 2005-147947 is an example of such a flux gate magnetometer and discloses a flux gate magnetometer with a magnetic sensor that excites a ring core composed of a magnetic detecting material up to a saturated magnetic field area by electrifying an exciting coil with an alternating signal to measure a magnetic flux density using symmetric property of a saturated magnetic flux density induced in the ring core. Japanese Patent Application Laid-Open Publication No. 1996-285929 discloses a magnetometer that supplies an excitation current from an oscillator to an exciting coil at a flux gate formed by winding the exciting coil and a detecting coil on a core to perform synchronous rectification of the output of the detecting coil with a synchronous rectification circuit. Recently, the flux gate magnetometer is expected to be applied to small devices such as a magnetic sensor for a portable compass. Japanese Patent Application Laid-Open Publication No. 2005-61969 discloses a flux gate magnetometer for accomplishing further improvement in accuracy in measurement of a magnetic force.
High accuracy and stability are required for a drive circuit driving an exciting coil of a magnetic sensor used in the flux gate magnetometer. For example, when the magnetic field measurement is performed in a plurality of spatial axis directions, a plurality of flux gate magnetometers is often used at the same time and, therefore, manufacturing variations must be reduced in each flux gate magnetometer in the case of mass production. When applying to small devices, a smaller component count is required and a chip area must not be occupied at the time of integration.